Distributed Fiber Optic Sensing Basics
The application of fibre optic is not limited to data transmission only and spreads over measurement of temperature, pressure and tension. Distributed sensing systems utilize optoelectronic devices coupled with optical fibre used as linear sensor. The temperature, pressure and tension affect the physical properties of the optical fibre material and laser light is used to measure these changes locally.
The light in the optical fibre scatters from the microscopic variations of the glass density naturally created during manufacturing. The typical size of these glass variations is less than the wave length of the light (approx. 1000 nanometers). The backscattered light contains three components of our interest – the elastically scattered (called Rayleigh) component with the same wavelength as incident light and two spectrally shifted components. The shift is determined by the resonance frequency of the fibre optic molecules lattice. The effect of changing the wavelength of backscattered light is called Raman scattering. There are two Raman scattered components – Stokes with longer wavelength and Anti-Stokes with shorter wavelength relative to Rayleigh component. The amplitude of the Anti-Stokes component is temperature-dependent. Measuring Raman components is a challenge since it is approximately one thousand times less than the amplitude of the Rayleigh scattering component. Special signal processing techniques should be applied to recover temperature values.
The next question is how to determine the location of temperature, pressure or tension measurement. This can be done by utilizing a well known OTDR, Optical Time Domain Reflectometry or OFDR, Optical Frequency Domain Reflectometry principles.
The OTDR is based on the measurement of the time difference between the events of sending a light pulse and receiving of backscattered components by transmitter-receiver optoelectronic device. Signal processing for Raman components is quite simple since OTDR operates in pulsed linear mode and does not affect much a backscattered components in terms of the phase and frequency. However there are limitations on spatial resolution and SNR (Signal-to-Noise-Ratio) due to pulse nature of the method.
The OFDR’s optoelectronics operate in a quasi-continuous emitting mode with a light’s wavelength changing linearly and amplitude periodically modulated. This allows to employ less expensive semiconductor laser diodes and electronic assemblies for signal averaging. Spatial resolution can potentially be improved as well since the method is not limited to the length of the pulse. The downside of the OFDR is related to difficulties in measurement of the Raman scattered light and complex signal processing.
Currently both OTDR and OFDR methods are used in DTS systems providing comparable performance and equipment cost.